[SI-LIST] Re: DC resistance of the Power Supply on PCB
- From: dgun@xxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 15 Dec 2005 22:51:39 -0800
From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
> To do that, don't measure resistance with a multimeter. Use the normal
> power supply for the board (or a lab supply with extra current limiting j=
ust
> in case) and an ammeter in one of its leads.
I think you will find that an ammeter will considerably load the power
supply. You could even get oscillations as the ammeter autoranges.
Better to use a precision 0.1 or 0.01 ohm resistor and measure the
voltage drop across it. It's much easier to make a high-impedance
voltmeter than a low-impedance ammeter.
--
Daniel
From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
>=20
> Hi Zhangkun,
>=20
> > As I know, the voltage of multimeter is about 12V.
>=20
> Maybe it takes a 12V battery, or runs on 12V external power ... but that
> definitely does not mean that it applies 12V to the device being measured
> for resistance. The meter may cut it down to something much smaller.
> Modern DVMs may use a voltage much less than 0.5V on the resistance range=
s,
> and something over a volt on the diode-test ranges.
>=20
> Anyway, these are the open-circuit ohmmeter voltages. They drop
> substantially when you connect any reasonable resistance. No good
> multimeter would apply a fixed 12V when measuring a resistance of 1 ohm, =
or
> in the dozens of ohms.
>=20
> > If I want to measure
> > the correct dc resistance between the power and ground, should I adjust
> > the voltage of the multimeter to that of measured power net?
>=20
> Well, that depends on why you want to measure the resistance.
>=20
> Active circuits can have very nonlinear characteristics, which can cause =
the
> resistance to change by orders of magnitude. If you want to know the
> resistance under operating conditions, you do need to use the normal supp=
ly
> voltage.
>=20
> To do that, don't measure resistance with a multimeter. Use the normal
> power supply for the board (or a lab supply with extra current limiting j=
ust
> in case) and an ammeter in one of its leads.
>=20
> If you are checking for faults, it may not matter if you use the same
> voltage. Although the reading might not correlate with the resistance at
> the normal supply voltage, it should be good enough for a go/no-go test (=
if
> you remember about observing polarity and charging bulk storage capacitor=
s).
>=20
> Regards,
> Andy
--=20
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